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The present invention relates to ultraviolet sources for curing ultraviolet sensitive inks and coatings, and more particularly to an ultraviolet curing system for printing presses which is zoned to allow for adjustment for various printing area widths.
Rotary offset printing presses reproduce an image on a substrate comprising successive sheets of paper, or a web of paper, by means of a plate cylinder which carries the image, a blanket cylinder which has an ink transfer surface for receiving the inked image, and an impression cylinder which presses the paper against the blanket cylinder so that the inked image is transferred to the substrate. Lithographic inks applied to the substrate can be partly absorbed and dry mainly by oxidation, penetration and absorption. Drying of lithographic inks can be enhanced by oxidation, penetration and absorption at somewhat elevated temperatures. Heat may be applied to the substrates by various means, see for example U.S. Pat. No. 5,537,925 which applies infra-red radiant heat and heated forced air flow to speed drying of such inks.
For multicolor printing, presses normally have a number of printing stations, one for each color. Dryers are often placed between printing stations to dry each image before the substrate enters the next printing station. At the end of the printing press, the substrates are normally delivered to a sheet stacker. A dryer is normally provided before the stacker to avoid any offsetting of images from substrates which are not completely dried.
In many applications, a protective or decorative coating is applied to printed substrates. As taught in U.S. Pat. No. 5,176,077, coating apparatus is available for installation in a conventional printing press. Such coatings should also be dried before the printed substrates are delivered to a stacker.
It is becoming more common to use ultraviolet, UV, curable inks and coatings in rotary offset printing presses and other types of presses, e.g. flexographic, screen printing, etc. UV coatings may be applied as protective or decorative coatings over images printed with other types of inks. UV inks and coatings have a number of advantages. They do not contain water or volatile hydrocarbon components and do not produce gases which have to be removed as normally occurs with other inks and coatings. Instead of drying by evaporation or oxidation, the UV curable materials polymerize in response to exposure to UV radiation.
UV curing units, commonly referred to as UV dryers, are available for installation in most printing presses. These available units generally use tubular quartz medium pressure mercury vapor lamps as a source of UV radiation. This type of lamp provides a fairly wide range of UV wavelengths which make them suitable for a variety of inks and coatings which may respond to different UV wavelengths. The conventional tubular lamps are positioned transversely across the width of the printing path. Multiple lamps spaced along the substrate travel path are used to increase total power and exposure, or dwell, time as necessary to achieve a good cure.
The mercury vapor lamps must be driven at relatively high power to generate a sufficient intensity of UV radiation to achieve rapid curing and to cure thick layers of UV inks and coatings. Such lamps also emit considerable energy in the visible and infrared frequencies which represents wasted energy and requires cooling fans to avoid overheating the lamps, the substrates and the printing presses. When printing a substrate of less width than the press capacity, all radiation, i.e. UV, IR, and visible from those portions of the lamps which extend beyond the edges of the substrate is wasted energy and is directed at press components and causes unnecessary aging and other damage to the press itself.
An ultraviolet curing unit according to the present invention includes a plurality of linear UV emitting devices spaced laterally from each other across a substrate travel path in a printing press and generally in alignment with the direction of the travel path. Each UV emitting device defines a curing zone. The UV emitting devices are individually controlled so that UV emitting devices for unneeded curing zones may be deactivated.
In a preferred form, each UV emitting device has a plurality of power settings, or a continuously adjustable power level, allowing adjustment according to the particular inks and/or coatings used in a particular printing job.
In another embodiment, the UV curing unit may include one UV lamp positioned transversely across the path of substrate travel. The transverse lamp initiates curing of UV curable inks and coatings before the printed substrate passes under the primary plurality of lamps.
In another embodiment, an infrared and/or hot air heater is positioned to heat the printed substrates before they are exposed to the UV emitting devices.